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Summary form only given. Optical coherent communications are superior to conventional intensity-modulation/direct detection (IM/DD) systems from the point of view of spectral efficiency. To reduce the laser phase noise effect, however, phase lock loop (PLL) circuits were indispensable in most of optical coherent detection. We propose a novel technique to detect the phase-shift of the lightwave based on injection-locking of a semiconductor laser. Using this technique, one can receive a phase-modulated signal without PLL. It is not seriously affected by the laser phase noise because of the frequency traceablity of injection-locking. The concept, to utilize an injection-locked laser as frequency-shift-keying (FSK) receiver had been presented, in which the laser was used as a resonant photodetector. On the other hand, the technique directly converts optical phase-shift to return-to-zero intensity symbol within the injection-locked laser, called differential-phase-to-intensity conversion (DPIC). It is all processed in the optical domain. Since DPIC responds only to the phase-shift, it is used as an optical differential-phase-shift-keying (DPSK) decoder.